The present invention relates to magnetite particles, magnetic iron oxide particles for magnetic toner, a process for producing the same and a magnetic toner using the same, and particularly it relates to magnetite particles, magnetic iron oxide particles for magnetic toner which are free from separation (falling-off) from toner particles, a process for producing the magnetic iron oxide particles, and a magnetic toner using the magnetic iron oxide particles, which shows a good fluidity and produces visual images having a higher definition upon development of electrostatic latent images.
As one of electrostatic latent image-developing methods, there has been widely known and generally utilized a developing method in which composite particles obtained by mixing and dispersing magnetic particles in a binder resin, without using a carrier, is used as a developer, i.e., a so-called one component magnetic toner developing method.
In recent years, accompanied with miniaturization of electrostatic copying machines and printing machines and realization of high performance such as a high speed operation thereof, there has been a strong demand for enhancing characteristics or properties of a magnetic toner as a developer, i.e., an excellent durability so as to obtain a stable developing performance even under severe use conditions. In addition, such a magnetic toner is required to have a high fluidity and produce visual images having a higher definition.
In general, the magnetic toner comprises magnetic particles and a binder resin. The magnetic particles are uniformly dispersed in respective toner particles. The magnetic particles which are exposed to a surface of each toner particle are apt to fall off from the toner particle due to frictional contact with other toner particles or the like. Especially, when the particle size of the toner is small, a large number of magnetic particles are exposed to the surface of the toner particle so that the magnetic particles are easily fallen off in the form of a fine powder from the surface of the toner particle. Such magnetic particles fallen off in the form of a fine powder cause various problems such as deterioration in environmental conditions within the copying or printing machines, disturbance of uniform triboelectric of the toner, deteriorated developability of the toner due to scattering of the magnetic particles upon the development of electrostatic latent images. For these reasons, it is intensely required to provide magnetic particles which are free from the falling-off from a surface of respective toner particles even when the magnetic particles are exposed to the toner surface, and which have an excellent durability.
With respect to the definition of visual images on the electrostatic latent image development, Japanese Patent Application Laid-open (KOKAI) No. 63-139367 (1988) describes that "in such a method using a dry developer, in order to produce visual images having a high quality, it is required that the developer exhibits a high fluidity and a uniform triboelectric property". Thus, the fluidity and uniform triboelectric property of the toner particles as a developer give a large influence on a quality of visual images produced. For this reason, a toner having a high fluidity and a uniform triboelectric property is highly required to obtain visual images having a higher definition.
Properties of a magnetic toner have a close relationship with those of magnetic particles which are mixed and dispersed in the magnetic toner. The fluidity of the magnetic toner highly depends upon a surface condition of each of the magnetic particles exposed to a surface of the magnetic toner. In this regard, as described in Japanese Patent Application Laid-open (KOKAI) Nos. 5-72801 (1993), 5-213620 (1993) and 7-101731 (1995) and the like, it is known that the magnetic particles having Si on a surface thereof enhance the fluidity of the magnetic toner.
The magnetic toner is occasionally exposed to an elevated temperature as high as not less than 150.degree. C. during the use. Even in such a case, the magnetic toner is required to have a stable color tone.
This fact has also been described in Japanese Patent Application Laid-open (KOKAI) No. 55-65406 (1980) as "In general, the magnetic particles for such a one component magnetic toner are required to have the following properties: . . . . iv) To have a practically usable blackness. Although a colorant can be contained in the magnetic toner, it is preferable that the particle itself has a black color without using any colorant. v) To have a high heat resistance. It is required that a color tone, especially a black color and an electromagnetic properties thereof are stable enough in the range of a temperature of 0 to 150.degree. C.
As well known in the art, unsuitable phenomenon that a color tone of the magnetic toner turns from black to brown is caused due to the fact that a blackness of magnetite particles is varied depending upon a content of Fe.sup.2+ and, therefore, there is a tendency that the more the content of Fe.sup.2+, the more the magnetite particles are excellent in blackness. However, when exposed to an elevated temperature as high as 150.degree. C., the Fe.sup.2+ in the magnetite particles is oxidized to Fe.sup.3+ so that magnetite is transformed into maghemite.
In addition, since the surface of the magnetic particle generally is hydrophilic, it is difficult to disperse the magnetic particles in a resin, thereby causing the difference in content of the magnetic particles between respective magnetic toner particles. As a result, the magnetic toner particles are likely to be magnetically coagulated with each other with the particles having a large content of the magnetic particles. Such magnetic particles having a hydrophilic surface have a large oil absorption.
In consequence, in order to obtain a magnetic toner capable of producing visual images having a high definition, it is required that the surface of the magnetic particle is hydrophobic and the oil absorption of the magnetic particles is small, in order to enhance a dispersibility of the magnetic particles in a resin.
As magnetite particles used as magnetic particles for a magnetic toner, there are known octahedral magnetic particles (Japanese Patent Publication (KOKOKU) No. 44-668 (1969)), spherical magnetite particles (Japanese Patent Publication (KOKOKU) No. 62-51208 (1987)), hexahedral magnetite particles (Japanese Patent Application Laid-open (KOKAI) No. 3-201509 (1991)) or the like.
There are also known magnetite particles having specific shapes, for example, magnetite particles having grain-shaped protrusions on surfaces thereof (Japanese Patent Application Laid-open (KOKAI) No. 5-345616 (1993)), polyhedral magnetite particles each having at least 10 faces on a surface thereof (Japanese Patent Application Laid-open (KOKAI) No. 5-43253 (1993)) or the like.
Hitherto, studies have been conducted with respect to processes for producing magnetite particles, which processes comprise adding Si during a reaction for the preparation thereof to improve properties of the magnetite particles. For example, there have been proposed a process for producing magnetite particles, which process comprises adding a silicon component to a ferrous salt-containing solution, mixing 1.0 to 1.1 equivalent of alkali based on Fe in the solution, thereafter carried out an oxidation reaction in the solution while maintaining the solution at a pH of 7 to 10, adding 0.9 to 1.2 equivalent of Fe based on the initially added alkali to the solution during the reaction to compensate a deficient amount of Fe, and carrying out an oxidation reaction in the solution while maintaining the solution at a pH of 6 to 10 (Japanese Patent Application Laid-open (KOKAI) No. 5-213620 (1993)), a process for producing spherical magnetite particles, which process comprises passing an oxygen-containing gas into an aqueous ferrous salt reaction solution containing ferrous hydroxide colloid obtained by reacting Fe.sup.2+ with 0.80 to 0.99 equivalent of alkali hydroxide, wherein water-soluble silicate is added to the reaction solution in an amount of 0.1 to 5.0 atomic % (calculated as an element Si) based on Fe (Japanese Patent Publication (KOKOKU) No. 3-9045 (1991)), a process for producing magnetite pigment, which process comprises adding not less than one equivalent of an alkaline solution to a ferrous salt solution, and during the oxidation reaction of the magnetite particles after completing the preparation of ferrous hydroxide, adding a hydroxy silicate-containing solution to the reaction solution in an amount of 0.5 to 3.0% by weight (calculated as a ratio of Si to Fe.sub.3 O.sub.4) at the time in which a ratio of Fe.sup.2+ to Fe.sup.3+ is in the range of 10 to 1.0 (Japanese Patent Publication (KOKOKU) No. 1-39864 (1989)), or the like.
There have also been proposed a process for forming a coating layer comprising a hydroxide or a hydrous oxide of Si, Al, Ti or the like to improve a heat resistance (Japanese Patent Application Laid-open (KOKAI) No. 8-133745 (1996)), a process for treating surfaces of particles with a hydrophobic treatment agent such as a coupling agent, silicone or a higher fatty acid to decrease an oil absorption of the particles (Japanese Patent Application Laid-open (KOKAI) No. 7-27773 (1995)), or the like.
Magnetic particles for the production of a magnetic toner which are free from the falling-off from the toner particles, and a magnetic toner using the magnetic particles which shows a good fluidity and produces visual images having a higher definition upon the development of electrostatic latent images, are most demanded at present. However, the afore-mentioned conventional processes fails to provide such magnetic particles and magnetic toner.
That is, the magnetite particles described in Japanese Patent Application Laid-open (KOKAI) No. 5-345616 (1993) have grain-shaped protrusions on a surface thereof. However, individual protrusions are too small so that the falling-off of the magnetite particles from the toner particles cannot be sufficiently prevented.
The magnetite particles described in Japanese Patent Application Laid-open (KOKAI) No. 5-43253 (1993) each have not less than 10 faces on a surface thereof. However, each face of the magnetite particle is not uneven but flat so that the falling-off of the magnetite particles from the toner particles cannot also be prevented.
The magnetite particles described in Japanese Patent Application Laid-open (KOKAI) No. 5-213620 (1993) is prepared by adding 1.0 to 1.1 equivalent of alkali based on ferrous iron during a first reaction. However, the obtained magnetite particles is of approximately a spherical shape and a large particle size distribution, thereby failing to produce magnetite particles having a uniform particle diameter.
The magnetite particles described in Japanese Patent Publication (KOKOKU) No. 3-9045 (1991) each have a spherical shape and is prepared at a pH as low as less than 8.0 without adjusting the pH during a first reaction.
Finally, the magnetite particles described in Japanese Patent Publication (KOKOKU) No. 1-36864 (1989) each have a octahedral shape whose faces are not uneven but flat so that the falling-off of the magnetite particles from the toner particles cannot also be prevented.
Accordingly, the technical problems of the present invention aim at providing magnetic iron oxide particles for a magnetic toner which are free from the falling-off from toner particles and the magnetic toner using the magnetic iron oxide particles which shows a good fluidity and a uniform triboelectric property so that visual images having a high definition can be produced upon the development of electrostatic latent images.
As a result of intense studies of the present inventors, it has been found that by carrying out two-step reaction comprising a first step of producing magnetite core particles by passing an oxygen-containing gas into an aqueous ferrous salt reaction solution containing ferrous hydroxide colloid prepared by reacting an aqueous ferrous salt solution with 0.80 to 0.99 equivalent of an aqueous alkali hydroxide based on a ferrous salt in the aqueous ferrous salt solution while heating the reaction solution in a temperature range of 70 to 100.degree. C.,
wherein during the reaction, either an aqueous solution of a water-soluble silicate in an amount of 1.0 to 8.0 atomic % (calculated as an element Si) based on Fe is dividedly added to the reaction solution in at least two parts or the aqueous solution of the water-soluble silicate at a rate of 0.5 to 5.0 wt %/minute based on a total amount of Si is continuously dropped to the reaction solution, and a pH of said aqueous ferrous salt reaction solution is adjusted to 7.5 to 9.5 when the degree of oxidation of Fe.sup.2+ (Fe.sup.3+ /whole Fe) during the reaction is not less than 20%; and
a second step of growing the magnetite core particles by adding not less than 1.00 equivalent of an aqueous alkali hydroxide solution based on residual Fe.sup.2+ present in the reaction solution after completing the first step, and passing an oxygen-containing gas into the reaction solution while heating the reaction solution in a temperature range of 70 to 100.degree. C.,
the thus-obtained magnetite particles having an average particle diameter of 0.05 to 0.50 .mu.m, which are of substantially a granular shape, more specifically a confeitos-shape wherein protrusions having round crests are present on a surface of each particle (i.e., each magnetite particle having a plurality of mountain-shaped protrusions with unsharpened crests on a surface of each particle), and the number of the mountain-shaped protrusions with unsharpened crests formed on a surface of each particle being 2 to 30 when viewed in projected plan, cause no falling-off from toner particles when produced by using such magnetite particles and are capable of providing the toner particles having an excellent fluidity. The present invention has been attained on the basis of this finding.